1. Field of the Invention
The present invention relates generally to a non-linear resistor which is suitable for use in a lightning arrestor, surge absorber and so forth. More particularly, the invention relates to a material for non-linear resistor which has excellent electrical and mechanical characteristics.
2. Description of the Background Art
Non-linear resistors have known electric characteristics to non-linearly increase current according to increasing voltage and whereby lower voltage in non-linear fashion. Such non-linear resistor are known as useful element for absorbing extraordinarily high voltage. Therefore, the non-linear resistors have been used in a lightning arrestor, surge absorber and so forth.
One of typical composition of a material for forming the non-linear resistor contains zinc oxide as primary component. The non-linear resistor material is further composed of relatively small amount of oxides, such as bismuth trioxide (Bi.sub.2 O.sub.3), cobalt oxide (Co.sub.2 O.sub.3), manganese dioxide (MnO.sub.2), antiminial oxide (Sb.sub.2 O.sub.3) and so forth. The composite material is prepared by mixing the compositions set forth above and by crystalizing. The composite material is then shaped into a desired configuration and fired at a given temperature. Such non-linear resistor material has a three-dimensional structure having ZnO crystal (10.degree.-cm) of 10 .mu.m surrounded by high resistance intergranular layer of less than or equal to 0.1 .mu.m thick, which intergranular layer contains Bi.sub.2 O.sub.3 as primary component.
As is well known, the intergranular layer filling up gaps between ZnO crystals has an electric property or characteristics to substantially and non-linearly decrease resistance according to increasing of changed voltage. When composition is held unchanged, voltage/current characteristics of each unit of crystal-insulative intergranular layer-crystal is considered to be substantially constant.
As set forth, the non-linear resistors have considered useful because of excellent electric or non-linear voltage/current characteristics. However, the conventional non-linear resistors were not satisfactory in mechanical characteristics, such as compression strength, bending strength and so forth because interest was concentrated to electric characteristics. Because of lack of mechanical strength, application of the non-linear resistor has been limited.